The invention relates to four-port hybrid junction networks which may be used as attenuators, phase shifters, signal combiners and splitters, mixers or modulators for high frequency or microwave apparatus.
These hybrid junctions are known in the prior art and belong to a class of hybrids in which ring and branch lines are used. A particular characteristic of these hybrid junctions is that there are two possible paths, from any port to any other port, differing so that signals arriving at a destination port oppose each other. This can be achieved by either insertion of a .lambda./2 long transmission line in one path or by use of an electrical reversal connection. The insertion of a .lambda./2 long transmission line in one path has an advantage of allowing a simple and inexpensive design but has the disadvantage of operating over a narrow frequency range due to the .lambda./2 long transmission line. The insertion of an electrical reversal connection performs better over a wider frequency range since the electrical reversal connection is, by itself, an item which is independent of the frequency. However, the designs involving an electrical reversal connection require a more complex type of arrangement.
U.S. Pat. No. 3,504.304 describes a hybrid network in which 4 sections of suitable transmission medium such as transmission lines, co-axial cables, waveguides or striplines are connected in a ring network with one arm connected so as to provide the needed 180.degree. phase shift. This U.S. patent further describes a compensation circuit, such as a variable reactance circuit, which is connected to each of the four junctions of the network to compensate the ring in a manner which increases the operating bandwidth and/or decreases the input voltage standing wave ratio(VSWR) with decreasing isolation between conjugate arms of the network.
U.S. Pat. No. 3,654,570 describes an improvement on U.S. Pat. No. 3,358,248 which is directed to a four port coaxial hybrid junction in which a pair of insulated centre conductors extend side-by-side for a quarter wavelength in a manner permitting this pair of centre conductors to be substantially co-axial with an outer conductor, the improvement being an improved means for coupling signals into and out of a hybrid junction device.
U.S. Pat. No. 3,497,832 describes a four port magic tee type hybrid apparatus with a pair of sub members .lambda./4 long having inner conductor portions co-axial with respective tubular intermediate conductor members which are aligned parallel to and equidistant from outer ground plane members. This U.S. Pat. No. 3,497,832 illustrates in FIG. 6 a stripline configuration which is analogous to the pair of co-axial type stub members illustrated in FIG. 4.